Attention Deficit Hyperactivity Disorder (ADHD) is a complex behavioral disorder that affects an estimated 5-10% of children in the United States. Recent research in ADHD suggests it is a complex behavioral phenotype that is the result of genetic heterogeneity with sample differences likely reflecting variation in relative proportions of various susceptibility genes, their corresponding brain correlates, and background gene effects. Gene discovery and subsequent elucidation of gene to brain to behavior pathways can be facilitated if etiological heterogeneity is reduced (for example, through use of genetic isolates and/or selection through multiplex families) or if the phenotype can be refined into familial components that may be more closely linked to specific underlying risk genes (i.e. endophenotypes). The identification of risk genes and associated ADHD subtypes may eventually lead to improved diagnostic methods and treatment interventions for children with ADHD. The goal of the proposed research is to investigate electroencephalographic (EEC) measures as a biological endophenotype in ADHD and to use it in gene mapping investigations. The proposed research makes use of the ongoing UCLA ADHD Genetic Study to collect EEC data on a set of 200 affected sibling pairs (ASPs), their parents, and unaffected siblings for this investigation. The specific aims of this grant are to: 1) identify specific EEG patterns as likely endophenotypes; 2) characterize the EEG endophenotype relative to behavioral and cognitive variation associated with ADHD; and 3) test association and linkage of EEG endophenotypes in ADHD. We believe that combining behavioral, cognitive, EEG and genetic data will identify the most powerful endophentoypes for gene mapping studies in ADHD and yield important information regarding the gene-brain-behavior pathway. Innovative statistical strategies for combining these data and identifying underlying traits that are part of the genetic liability for ADHD will be used. The current study benefits greatly from this ongoing data collection because ASPs are routinely assessed using structured diagnostic interviews, neuropsychological testing, and blood draws for genotyping investigations. We are thus able to collect EEG data at a very low cost in a well characterized sample of ADHD ASPs upon which linkage studies (and candidate gene association work) are readily available.